Lithography is a technology that has facilitated an increase in functionality of devices and improved yield while reducing size, failure rate, and cost. Improvements in lithographic technologies have led to increasingly smaller scale devices, circuits, and features.
Contact-mask photolithography is another technology for fabricating microstructures and is commonly used, for example, in custom manufacturing of RF (Radio Frequency) devices, compound semiconductors, and so forth. In contact-mask photolithography a desired pattern is created on a mask (also referred to as a photomask) as patterns in a metal film. The patterns on the metal film can be created in various ways, such as using laser direct-write systems or scanning-electron-beam-lithography systems.
A conventional contact-mask photolithography process is illustrated in FIG. 1a-1d. In contact-mask photolithography, the mask 110 is brought into contact with a substrate 120 overcoated with photoresist. When the mask and the substrate are illuminated with light 130 from an appropriate light source, such as an ultra-violet wavelength light emitting source, the metal regions on the mask block the light from reaching the underlying photoresist. The light passes through regions of the mask that do not have metal (due to the patterning described above) and exposes the underlying photoresist layer. After exposure, the mask is removed from the substrate and the photoresist on the substrate is developed. In other words, the regions of the photoresist that were exposed to light can be removed, such as by a solvent. Regions 125 not exposed to the light can remain. This process is referred to as a positive-tone photoresist process. In an alternate negative-tone photoresist process, the regions that were exposed to light remain while a solvent removes the regions that were not exposed to the light.
Contact-mask photolithography technologies have been in use for a number of years. However, the technology has drawbacks. Some challenges associated with contact-mask photolithography are the fabrication of the template and durability of the mask. High resolution mask patterning is commonly performed with electron beam lithography or focused ion beam patterning. However, at smaller resolutions the throughput for these techniques can be very low. For example, an electron beam lithography method can take a significant amount of time to cover a 15 cm diameter substrate with a dense pattern of 10 nm features. If the desired pattern is altered, a new mask is made. While experimenting with new devices, alteration of the design may not be uncommon or infrequent. This expensive and time-consuming process patterning process may thus be a disadvantage for prototyping as well as for low-volume production of devices.
Optical patterning of masks has been demonstrated, but conventional photolithography is limited in resolution to relatively large features (i.e. generally greater than approximately 100-200 nm (i.e., ˜1-2 micron) feature sizes). Wear of the mask can also be a significant concern. Creation of the masks can thus be slow, difficult and expensive. However, a mask, once created, can be used repeatedly for a large number of substrates, thus offsetting the time and expense in creating the mask.
Creating sub-micron features on a mask while achieving a good contact for transfer of sub-micron features onto a photoresist layer can therefore be challenging.